Approximately 40 murine genes have been shown to control resistance to various virus infections, but only one of these, the Mx gene has so far been cloned and sequenced. In mice, resistance to flavivirus-induced morbidity and mortality is inherited as an autosomal dominant trait. Although the molecular basis for the observed functional differences between the resistant and susceptible alleles of the Flv gene is not known, our recent data suggest that the product of the Flv gene functions at the level of flavivirus RNA synthesis. Data from both yellow fever virus and dengue virus outbreaks have suggested the possible existence of a human Flv homolog. A previous multipoint linkage analysis mapped the flavivirus resistance gene, Flv, within a 0.45 cM segment on mouse chromosome 5 between loci D5Mit408 and D5Mit242. Tight linkage between the D5Mit159 locus and the Flv gene was observed. We propose to positionally clone the Flv gene. BAC clones from the D5Mit159 region will first be selected and aligned and then the transcriptional units within these BAC clones will be identified. Complete cDNA sequences of candidate genes will then be obtained by direct selection and by exon trapping. The cDNAs obtained will be sequenced and primers designed from their termini will be used to amplify cDNAs for the alleles from congenic resistant and susceptible mouse cells. The sequences of pairs of "resistant" and "susceptible" cDNAs will be determined and compared and the proteins expressed from them will be functionally tested in an in vivo assay for a dominant, negative effect on flavivirus replication. Studies of the mechanism of this natural viral resistance will be initiated after the identification of the Flv gene.